What do you need to represent?

# advent-of-code

[orangy]

What do you need to represent?

[karelpeeters]

A grid with positive x and y coordinates where the value at (x, y) depends on the values at (x-1, y) and (x, y-1). It also needs to be lazy.

[orangy]

Is size known?

[karelpeeters]

Nope.

[karelpeeters]

Otherwise making it lazy wouldn't be necessary.

[orangy]

I would try a linear Int array with diagonal encoding

[orangy]

0136
247
58
9

Indicies would go like this

[orangy]

Calculating index of down or right cell is pretty trivial, and you can resize it like normal array (x2)

[karelpeeters]

But then if you access (0, 100) you have to calculate a bunch of excess stuff, right?

[orangy]

allocate yes, calculate no

[karelpeeters]

I was going to say that the ratio for this challenge is 7/713, but we do a BFS on it so it should pretty much be even.

[karelpeeters]

I'll give it a shot!

[karelpeeters]

https://math.stackexchange.com/a/1067928/286346

[karelpeeters]

Finally got it: i = (x + y) * (x + y + 1) / 2 + y. It's too early for math!

[karelpeeters]

But the array works! It's about 30% faster than the lists.

[orangy]

30% is quite an improvement!